HP 8517B - Manual (Page 48)

HP 8517B S-Parameter Test Set Manual 3-10
Changing Signal Path States After System
Calibration
Changing an internal attenuator or any external equipment after completing
the calibration means the measurement results cannot be specified. You must
use your own estimation of the potential error contribution due to the
change.
For example, when the port attenuation is changed with correction ON, the
message
CAUTION: CORRECTION MAY BE INVALID displays. You must judge
whether the error is tolerable in the particular application and how to
compensate for the change.
The application question is: “Does increasing the signal level during
calibration improve the calibration enough to risk a possible increase in error
contributions when you change the setup?”
The only reason for changing an internal attenuator or external equipment
between calibrations and measurements is to maximize the signal level
under both conditions, thus minimizing uncertainty due to noise.
Many factors enter into a setup-change decision, for example:
•
is it more accurate to calibrate at a low signal level without changing the
setup, or
•
is it more accurate to change the setup to optimize levels for both the
calibration and measurement
Changing attenuators at Port 1 or Port 2 does not change the test set
mismatch, directivity, or isolation characteristics severely. It does change the
frequency response magnitude and phase, however. The difference between
frequency response calibration and measurement can be normalized by using
HP 8510C trace memories.
Changing the Test Port
Attenuators If only the attenuators at Port 1 or Port 2 are changed, use the following
procedure to minimize errors:
1. Connect a short, or a thru, and set Port 1 and Port 2 attenuators for the
best IF signal levels during calibration.
2. Perform the appropriate measurement calibration.
3. Connect the correct calibration standard, set Port 1 and Port 2
attenuators to the value required for operating the test device.